In view of dwindling supplies of crude oil, enhanced oil recovery (EOR) techniques are receiving renewed attention.
Typically, oil is produced using the natural pressure of an oil reservoir to drive the crude into the well bore from where it is brought to the surface with conventional pumps. After some period of production, the natural pressure of the oil reservoir decreases and production dwindles. In the 1940s, producers incorporated secondary recovery by utilizing injected water, steam and/or natural gas to drive the crude to the well bore prior to pumping it to the surface.
Once the easily extracted oil already has been recovered, producers may turn to tertiary or enhanced oil recovery (EOR) techniques. One known such EOR technique is high-pressure CO2 injection, which helps to repressurize the oil reservoir. The high-pressure CO2 also acts as a solvent, dissolving the residual oil, thereby reducing its viscosity and improving its flow characteristics, allowing it to be pumped out of an aging reservoir.
One difficulty with the use of CO2 to increase oil production is that it requires large quantities of CO2, and the availability of such large quantities of CO2 is limited.
CO2 from natural sources can be utilized, but generally requires the natural source to be in the proximity of the oil reservoir to avoid the construction and use of pipelines, which could make such use uneconomical.
Use of CO2 from combustion sources (such as power plants) has also been considered (see, for example, U.S. Pat. No. 7,299,868 and publications cited therein), but the separation of CO2 from the combustion gases is difficult and generally not considered economical.
More recently, CO2 from synthesis gas production operations has been considered for use in EOR. See, for example, U.S. Pat. No. 7,481,275. Synthesis gas production operations include, for example, catalytic gasification and hydromethanation processes, non-catalytic gasification processes and methane reforming processes. These processes typically produce one or more of methane, hydrogen and/or syngas (a mixture of hydrogen and carbon monoxide) as a raw gas product, which can be processed and ultimately used for power generation and/or other industrial applications. These processes also produce CO2, which is removed via acid gas removal processes, as is generally known to those of ordinary skill in the relevant art. Historically, this CO2 has simply been vented to the atmosphere but, in view of environmental concerns, capture and sequestration/use of this CO2 is becoming a necessity. EOR is thus a logical outlet for CO2 streams from synthesis gas production operations.
At least one such synthesis gas production operation which utilizes a CO2 by-product stream for EOR currently exists at the Great Plains Synfuels Plant (near Beulah, N. Dak. USA). At this facility, coal/lignite is gasified to a synthesis gas stream containing carbon dioxide, which is separated via a solvent-based acid gas removal technique. The resulting CO2 stream (which is greater than 95% pure) is compressed and transported via a 205-mile supercritical CO2 pipeline to oil fields in Canada for use in EOR operations. This operation is described in more detail in Perry and Eliason, “CO2 Recovery and Sequestration at Dakota Gasification Company” (October 2004) (available from www.gasification.org), and on the Dakota Gasification Company website (www.dakotagas.com).
A disadvantage in this operation is the pipeline, as supercritical CO2 is considered a hazardous material. The construction, permitting, operation and maintenance of a supercritical CO2 pipeline, particularly one as long as 205 miles, is expensive. A more advantageous way to get the CO2 from the synthesis gas operation to the EOR site would, therefore, be highly desirable.
A variation on CO2 injection is to use a sour CO2 stream containing hydrogen sulfide. The presence of H2S in the CO2 stream has been shown to potentially lower the minimum miscibility pressure for a particular oil, and thus potentially improve the effectiveness of CO2-based EOR.
A sour CO2 stream, and particularly a supercritical CO2 stream containing hydrogen sulfide, would be considered hazardous and thus not transportable over standard pipelines.
It would, therefore, be highly desirable to integrate synthesis gas production processes with EOR processes in a way that minimizes the release of CO2 into the atmosphere (maximizes capture and sequestration of CO2), reduces the need for long CO2 transport pipelines, provides a sour CO2 stream for EOR, and improves the overall integration, efficiency and economics of the two processes. The present invention provides such an integration.